Who is the storngest character of the series?

That depends completely on what you mean by that. Break has the strongest Chain in terms of offensive power against Baskerville, and physically he was able to fend off Fang and his giant sword with a thin blade that should have broken.
Sheryl has the most strength in other aspects (as she could embed a paper harisen in a stone pillar without over exerting herself), and because of this most people in Pandora are afraid of her.
Not to mention the Five Black Winged Chains of Glen Baskerville are powerful in their own respects, their strength being comparable to Oz's own.
Overall, Jack had the most power. Oz is capable of destroying not only the world, but Chains, Baskervilles and people too with his powers. Therefore Jack had most power with his Chain, it just wasn't his own.
The main point is that every character has their own aspects to empower them :)